Three-dimensional particle tracking velocimetry measurements of flow around freely falling ice-particles*

Aerodynamics of an ice-particle is influenced by its fall speed, orientation and shape. Three dimensional particle tracking velocimetry measurements were performed for freely falling 3D printed ice-particle analogues and their aggregates in a vertical tank of quiescent water-glycerine mixture. Plate-like and columnar ice-particle crystals along with their aggregates were used in the present investigation. 3D printed analogues of ice-particle aggregates closely mimic complex shapes of hydrometeors in clouds and atmosphere due to the varying  number and orientation of the constituent particles forming the aggregates. Flow physics and the fall attitude of particles and their aggregates have been investigated. The critical Reynold numbers (Re) for the onset of unsteadiness in the flow differs due to the flow separation behaviour at the edges of singular particles and those at the sides of aggregates. The wake is a signature of geometry of the particle and its fall speed. Three dimensional wakes were noted with a larger wake width for the aggregates. Three regimes: (i) Steady, (ii) Shedding, (iii) Chaotic regimes were identified for the falling particles based on nature of the wake at various Re. The flow field and vortical structures are compared for the particles and their aggregates. The flow characteristics of the wake and the fall attitude of hydrometeors are coupled. The fall trajectory of the particles along with the  nature of orientation in both the steady and unsteady regimes was compared with the behaviour of their aggregates. The crystals oriented themselves with approximately the maximum projected cross-section facing the fall direction, while aggregates had a less predictable orientation, which varied with Re in some cases. The crystals flutter, rotate and spiral during the descent depending on their geometry and terminal velocity.